Thick film getter paste compositions for use in moisture control

ABSTRACT

The invention is directed to a screen-printable getter composition comprising: (a) glass frit; dispersed in (b) organic medium. The invention is further directed to a screen-printable thick film getter composition comprising: (a) glass frit; and (b) desiccant material; dispersed in (c) organic medium. 
     The present invention further relates to a getter composition utilizing low-softening temperature glasses comprising, based on weight %, 1-50% SiO 2 , 0-80% B 2 O 3 , 0-90% Bi 2 O 3 , 0-90% PbO, 0-90% P 2 O 5 , 0-60% Li 2 O, 0-30% Al 2 O 3 , 0-10% K 2 O, 0-10% Na 2 O, and 0-30% MO where M is selected from Ba, Sr, Ca, Zn, Cu, Mg and mixtures thereof. The glasses described herein may contain several other oxide constituents that can substitute glass network-forming elements or modify glass structure.

This application is a divisional of a U.S. patent application Ser. No.10/712,670, filed on Nov. 13, 2003 now U.S. Pat. No. 7,438,829.

FIELD OF THE INVENTION

The invention is directed to thick film getter paste composition for usein controlling moisture in moisture sensitive devices.

BACKGROUND OF THE INVENTION

The invention describes a thick film getter composition for use incontrolling moisture. Many environments are susceptible to unwantedmoisture and this is especially so in the case of various electronicdevices. In an effort to control unwanted moisture, the concept of usinggetter has been known for many years. Getters are substances which areadded to a system or mixture to consume or inactivate traces ofimpurities.

One approach to minimizing the deleterious effects of moisture hasinvolved the enclosure of moisture sensitive devices in a barrier toseparate the active materials from oxygen and moisture. This approachhas had some success, but it does not always adequately address theproblems caused by even those small amounts of moisture trapped withinthe enclosure or diffusing into the enclosure over time.

The use of getters for controlling moisture within a sealed enclosurefor an electronic component or device is well known. These sealedenclosures are designed to protect sensitive electronic components anddevices from the outside environmental contaminants, including moisture.However, some electronic devices are highly sensitive to moisture andrequire moisture control to very low levels. At they same time, theseelectronic devices require that the getter composition be easily appliedand processed while demonstrating that the processed compositionsufficiently adheres to the desired substrate. The present inventionprovides a superior composition to accommodate these requirements.

The prior art materials associated with getters and their use inelectronic applications have been described below. Typically, the gettermaterials not screen-printable compositions and are comprised of adesiccant material (i.e., zeolite, silica gel, etc.) and a binder. Thebinder may be organic or inorganic. The following illustrate the stateof the prior art.

U.S. Pat. No. 5,244,707 to Shores discloses a sealed enclosure of anelectronic device which incorporates a coating or adhesive withdesiccant properties. The coating or adhesive comprises a protonatedalumino silicate powder dispersed in polymer.

U.S. Pat. No. 5,591,379 to Shores teaches a composition of matter usefulas a desiccant in a hermetic electronic device, comprising a powderdispersed in a binder, wherein said powder is selected from the groupconsisting of zeolite molecular sieves, activated alumina, silica gel,alkaline earth oxide, and alkali metal carbonate; said binder is acontinuous matrix of porous glass or porous ceramic; and said powder tobinder volume ratio is 0.001-2. The glasses disclosed for use as abinder must be made porous by creating channels for water vapor topenetrate. This may be done by various techniques known in the art, suchas the use of blowing agents, fast evaporation of water or other gasesduring formation, fast decomposition of metalloorganic polymers and lowtemperature or incomplete sintering.

U.S. Pat. No. 1,626,682 to MacRae discloses a multi-pixel flat paneldisplay means which include spaced apart first and second electrodes,with a patterned solid material layer in contact with one of theelectrodes, exemplarily between the two electrodes. The patterned layer(the web) includes a multiplicity of apertures, with at least oneaperture associated with a give pixel. In the aperture is disposed aquantity of a second material, exemplarily, a phosphor in the case of aflat panel field emission display, or a color filter material in thecase of a liquid crystal display. The web can facilitate second materialdeposition by means of screen printing. The web also can facilitateprovision of spacer structure between two electrodes, and can includegetter or hygroscopic material.

U.S. Pat. No. 5,401,706 to Fischer teaches a process for making adesiccant-coated substrate capable of being used at temperatures over150 degrees Fahrenheit, the desiccant being in the form of particles andthe particles having pores and being adhered to the substrate by abinder, the coated substrate being sufficiently flexible and the coatinghaving sufficient adherence to the substrate so that the coatedsubstrate can be formed into corrugated shapes, the desiccant particlesin the coated substrate having at least 60% of their original adsorptioncapacity and the binder having good breathability; said processcomprising the steps: (a) forming an aqueous suspension comprisingparticles of one or more desiccants, a water-based organic binder, asuspending agent to help maintain the desiccant particles in suspension,and an organic pore-clearing agent at least some of which enters atleast some of the pores of the desiccant particles; (b) depositing thesuspension on the substrate; and (c) causing the binder of the depositedsuspension to set so that the deposited desiccant particles adhere tothe substrate and causing at least some of the pore-clearing agent toleave the pores of the desiccant particles to prevent the binder fromoccluding at least some of the pores of the adhered desiccant particles,thereby to form a desiccant-coated substrate capable of being used attemperatures over 150 degrees Fahrenheit and of sufficient flexibilityand having a coating having sufficient adherence to the substrate sothat the desiccant-coated substrate can be formed into corrugated shapesand in which the desiccant particles in the coated substrate have atleast 60% of their original adsorption capacity and in which the binderhas good breathability.

The present invention provides a screen-printable thick film gettercomposition comprising glass which may be used to promote densificationat low densification temperature of 400-650° C. and to provide increasedadhesion between the thick film and the substrate. Furthermore, thecomposition of the present invention may be utilized in display deviceswhich often require humidity control to levels below about 1000 ppm andsometimes below even 100 ppm.

SUMMARY OF THE INVENTION

The invention is directed to a screen-printable getter compositioncomprising: (a) glass frit; dispersed in (b) organic medium. Theinvention is further directed to a screen-printable getter compositioncomprising: (a) glass frit; and (b) desiccant material; dispersed in (c)organic medium.

The present invention further relates to a getter composition utilizinglow-softening temperature glasses comprising, based on weight %, 1-50%SiO₂, 0-80% B₂O₃, 0-90% Bi₂O₃, 0-90% PbO, 0-90% P₂O₅, 0-60% Li₂O, 0-30%Al₂O₃, 0-10% K₂O, 0-10% Na₂O, and 0-30% MO where M is selected from Ba,Sr, Ca, Zn, Cu, Mg and mixtures thereof. The glasses described hereinmay contain several other oxide constituents that can substitute glassnetwork-forming elements or modify glass structure.

DETAILED DESCRIPTION OF INVENTION

The thick film getter composition of the present invention is ascreen-printable ceramic composition, which may be formed on asubstrate. For example, a glass substrate acting as a lid in variouselectronic display devices. The thick film getter acts as awater-absorbent at various environmental conditions of a few thousandsppm to a few ppm of humidity levels. The present invention is notlimited to display device applications, but can be used for any othermicroelectronic device applications where the device lifetime can bedegraded by or sensitive to the presence of moisture.

Furthermore, the present invention relates to the use ofglass-containing composition, wherein no desiccant material is required,as a thick film getter or as an additive to promote densification ofresultant thick films and adhesion between the thick film and substrate.The thick film getter, after firing at 400-650° C., demonstrates gooddensification and compatibility particularly with glass lid substratesassociated with display device applications. The thick film getter pasteis primarily intended to be used for various display applicationsincluding organic light emitting diodes (OLED) and electroluminescence(EL), where lifetime of the display devices are strongly related to thecontrol of humidity and other harmful gases. The display applicationsdiffer from other hermetic microelectronic devices in that the moisturegettering is working at highly moisture-sensitive or gas-sensitiveenvironments. The display devices often require humidity control tolevels below about 1000 ppm and sometimes below even 100 ppm.

The main components of the thick film getter composition are a glasspowder dispersed in an organic medium. The organic medium is comprisedof organic polymeric binder and volatile organic solvent. In mostapplications, the composition may further comprise a desiccant material(for example, zeolite) or other material which absorbs impurities, suchas hydrogen or other gases. Usually, the desiccant material is the mainfunctional part of the thick film getter, which determines itsperformance of water absorption. Generally, the capacity of waterabsorption proportionally depends on the relative content of thedesiccant material. Primarily, glass provides a low densificationtemperature of 400-650° C. and good adhesion between thick film andsubstrate. Firing temperature is limited by the choice of lid substrate,since the thick film paste is supposed to adhere to substrate throughfiring. For example, firing temperature needs to be below 650° C. if atypical glass substrate based on soda lime silicates is selected. Firingabove 650° C. with the glass substrate may induce warping or distortionof the glass substrate. In case of LTCC or metal-based substrates, atemperature above 650° C. may be allowed.

Adhesion between the getter film and substrate is improved by theexistence of the low softening glass. Glass can help relieve interfacestress by facilitating penetration of glass into voids of substrates viaviscous flow during firing. Mechanical locking is likely to be thedominant mechanism for adhesion between thick film and substrate.However, the adhesion mechanism depends on the choice of substrate.

In the present invention, the glass does not necessarily form acontinuous matrix in unfired or fired structure. Before firing, theglass is dispersed in the organic media to form a viscous paste. Uponfiring, glass tends to wet the surface of desiccant particles or thesurface of the substrate and be densified by viscous flow. Theevaporation of organic media occurs before the actual consolidation ofsolid particles. The final fired structure must depend on the relativecontent of solids.

The glass itself is not porous and it is not necessary to produce theglass by a special fabrication process, such as fast evaporation ofwater or glasses, the use of blowing or bubbling agents, any chemicalhollow process and low temperature firing, to intentionally generatepores in the glass. Rather, it is important to achieve a highly dense oressentially void-free structure with the proper choice of glasschemistry.

The hardness and mechanical strength of the film primarily depend on thecontent and type of glass. The mechanical integrity is assumed to beimportant because weak film structure can be damaged by an externalmechanical shock. It is not necessary to maintain high levels ofporosity for better performance of moisture absorption, particularlywhen a highly moisture sensitive device working at a humidity levellower than 1000 ppm is used. In the device at humidity levels below 1000ppm, the glass paste itself (without desiccant material) may be utilizedas a functional getter. A proper choice of glass with strong hygroscopicnature is required. The pure glass getter film provides a glasscomposition which is not essentially porous and further allows strongintegrity with the substrate without any defects.

The thick film getter composition, as described herein, is bonded tosubstrate by depositing the thick film composition onto the substrateand firing the substrate. Depending on device structure and requiredperformance, thickness of the getter film may be controlled by thesequential depositions of the paste. A typical thickness of the thickfilm getter after single printing and subsequent firing ranges from 10μm to 25 μm. An excessive thickness does not need to be considered aslong as strong absorption of moisture is required.

Although screen printing is expected to be a common method for thedeposition of thick film getter, any other conventional methodsincluding stencil printing, syringe dispensing or other deposition orcasting techniques may be utilized.

This invention is based on the finding that thick film gettercompositions (sometimes referred to as “pastes”) can be made bycombining low softening temperature glasses with or without desiccantmaterial in an organic medium. The composition of the invention isdescribed in further detail below.

Inorganic Composition

The inorganic composition of thick film getter paste is comprised of aglass frit powder and may further comprise a desiccant material.

The glass frit powder is essential to the composition to promoteadhesion to the substrate and densification of the composition uponprocessing. In addition, the glass itself can act as a getterparticularly in a highly moisture-sensitive devices at a humidity levellower than 1000 ppm. Therefore, adequate levels of glass must be withinthe range of 10 wt % to 100 wt %, based on solid wt. %.

The present invention relates to low-softening temperature glassescomprising, based on weight %, 1-50% SiO₂, 0-80% B₂O₃, 0-90% Bi₂O₃,0-90% PbO, 0-90% P₂O₅, 0-60% Li₂O, 0-30% Al₂O₃, 0-10% K₂O, 0-10% Na₂O,and 0-30% MO where M is selected from Ba, Sr, Ca, Zn, Cu, Mg andmixtures thereof. The glasses described herein may contain several otheroxide constituents. For instance ZrO₂ and GeO₂ may be partiallyincorporated into the glass structure. Hygroscopic lithium-basedsilicates or phosphate glasses may also be applied for the getterpurpose. Furthermore, the glass compositions of the present inventionare not essentially porous glasses. Therefore, the glass compositions ofthe present invention are not required to be made porous by creatingchannels for water vapor to penetrate.

High contents of Pb, Bi or P in glass provide a very low softening pointthat allow thick film pastes to become dense below 650° C. These glassesare not crystallized during firing, since the elements tend to providegood stability of glass and a high solid solubility for other glasselements. In addition, the existence of a large content of phosphorousmay be useful in gettering moisture without affecting the degree ofdensification. Li-based silicate glasses may also be used in thisapplication, since they are sensitive to moisture and can be densifiedat low temperature below 650° C. However, the content of Li may need tobe optimized for full densification before crystallization happens.

Other glass modifiers or additives may be added to modify glassproperties for better compatibility with a given substrate. For example,Temperature Coefficient of Expansion (TCE) of the glass may becontrolled by the relative content of other glass components in thelow-softening temperature glasses.

The glasses described herein are produced by conventional glass makingtechniques. More particularly, the glasses may be prepared as follows.The glasses were typically prepared in 500-2000 gram quantities.Typically, the ingredients were weighted then mixed in the desiredproportions and heated in a bottom-loading furnace to form a melt inplatinum alloy crucibles. Heating was typically conducted to a peaktemperature (1100-1400° C.) and for a time such that the melt becomesentirely liquid and homogeneous. The glass melts were quenched by acounter rotating stainless steel roller to form a 10-20 mil thickplatelet of glass. The resulting glass platelet was then milled to forma powder with its 50% volume distribution set between 2-5 microns. Theglass powders were then formulated with filler and organic medium into athick film composition (or “paste”). The glass composition of theinvention at hand, is present in the amount of about 5 to about 76 wt.%, based on total composition comprising, glass, desiccant material, andorganic medium.

Various desiccant materials such as molecular sieves (or zeolites),alkaline earth metal oxides, metal oxides, sulfates, chlorides, bromidesmay be selected for getter paste applications. Various types of zeolitesare known to be the best candidates in terms of capacity of waterabsorption due to their intrinsic nature of porous structure.

Zeolites are materials that absorb moisture by physical absorption andmay be naturally or synthetically derived. Natural zeolites are hydratedsilicate of aluminum and either sodium or calcium or both, of the typeNa₂O, Al₂O₃, xH₂O, and xSiO₂. Synthetic zeolites are made either by agel process or a clay process, which forms a matrix to which the zeoliteis added. Both natural and synthetic zeolites may be used in the presentinvention. Well known zeolites include chabazite (also referred to aszeolite D), clinoptilolite, erionite, faujasite (also referred to aszeolite X and zeolite Y), ferrierite, mordenite, zeolite A, and zeoliteP. Detailed descriptions of the above-identified zeolites, as well asothers, may be found in D. W. Breck, Zeolite Molecular Sieves, JohnWiley and Sons, New York, 1974, hereby incorporated by reference. Forexample, type 3A, 4A and 13X zeolites all have the ability to adsorbwater molecules and are presently preferred as the adsorbent molecularsieve for making the new moisture getters. Such zeolites comprise Na₂O,Al₂O₃ and SiO₂. Certain adsorbent getters can adsorb gaseouscontaminants in addition to moisture, such as gaseous H₂ and O₂. Anexample of a commercially available, solid getter tablet based onzeolite technology that can be made to adsorb organics, as well asmoisture is described in European Patent Application No. WO 02/43098 A1by Synetix.

The amount of dessicant material added to the composition should bedetermined based on the necessary capacity to absorb moisture, asdictated by the application. The approximate volume % ratio of dessicantto glass frit ranges from 0 to about 15. The preferred volume % ratio ofdessicant to glass frit ranges from 2.1 to 10.

Organic Medium

The organic medium in which the glass and desiccant materials aredispersed is comprised of the organic polymeric binder which isdissolved in a volatile organic solvent and, optionally, other dissolvedmaterials such as plasticizers, release agents, dispersing agents,stripping agents, antifoaming agents and wetting agents.

The solids are typically mixed with an organic medium by mechanicalmixing to form a pastelike composition, called “pastes”, having suitableconsistency and rheology for printing. A wide variety of inert liquidscan be used as organic medium. The organic medium must be one in whichthe solids are dispersible with an adequate degree of stability. Therheological properties of the medium must be such that they lend goodapplication properties to the composition. Such properties include:dispersion of solids with an adequate degree of stability, goodapplication of composition, appropriate viscosity, thixotropy,appropriate wettability of the substrate and the solids, a good dryingrate, good firing properties, and a dried film strength sufficient towithstand rough handling. The organic medium is conventional in the artand is typically a solution of the polymer in solvent(s).

The most frequently used polymer for this purpose is ethyl cellulose.Other examples of suitable polymers include ethylhydroxyethyl cellulose,wood rosin, mixtures of ethyl cellulose and phenolic resins,polymethacrylates of lower alcohols, and monobutyl ether of ethyleneglycol monoacetate, as well as others known to those skilled in the art,may also be used.

The most widely used solvents found in thick film compositions are ethylacetate and terpenes such as alpha- or beta-terpineol or mixturesthereof with other solvents such as kerosene, dibutylphthalate, butylcarbitol, butyl carbitol acetate, hexylene glycol and high boilingalcohols and alcohol esters. In addition, volatile liquids for promotingrapid hardening after application on the substrate can be included inthe vehicle. The preferred mediums are based on ethylcellulose andβ-terpineol. Various combinations of these and other solvents areformulated to obtain the viscosity and volatility requirements desired.

The ratio of organic medium in the thick film composition to theinorganic solids in the dispersion is dependent on the method ofapplying the paste and the kind of organic medium used, and it can vary.Usually, the dispersion will contain 50-80 wt. % of inorganic solids and20-50 wt. % of vehicle in order to obtain good coating. Within theselimits, it is desirable to use the least possible amount of bindervis-à-vis solids in order to reduce the amount of organics which must beremoved by pyrolysis and to obtain better particle packing which givesreduced shrinkage upon firing. The content of the organic medium isselected to provide suitable consistency and rheology for casting,printing, such as silk screen printing or ink-jet printing, or coatingby spraying, brushing, syringe-dispensing, doctor blading, and the like.

In addition, the proportion of dispersing organic medium in the getterpaste may control the thickness of the solidified layer of absorbentgetter. For example, a thick film paste with minimized organic mediumresults in formation of a thicker getter layer (such dispersions aresubject to shear-thinning and hence become thinner as the dispersion isworked on the surface).

Application of Thick Films

The thick film getter is typically deposited on a substrate, such asglass, low temperature co-fired ceramics (LTCC) or silicone, that isimpermeable to gases and moisture. The substrate can also be a sheet offlexible material, such as an impermeable plastic such as polyester, forexample polyethylene terephthalate, or a composite material made up of acombination of plastic sheet with optional metallic or dielectric layersdeposited thereupon. In one embodiment, the substrate can be transparent(or semitransparent) to enable light to enter into the encapsulatedregion or to enable light to be emitted from the encapsulated regionthrough the substrate.

The deposition of thick film getter is performed preferably by screenprinting although other deposition techniques such as stencil printing,syringe dispensing or other deposition or casting techniques can beutilized. In case of screen-printing, the screen mesh size controls thethickness of deposited thick film.

The deposited thick film is dried at 100-120° C. in an oven to removevolatile organic medium, and then firing is carried out at 400-650° C.in a standard thick film conveyor belt furnace or in a box furnace witha programmed heating cycle forming a fired article. As used herein, theterm “firing” means heating the article in an oxidizing or inertatmosphere such as air, nitrogen or argon to a temperature and for atime sufficient to volatilize (burn-out) the organic material in thelayers of the assemblage and to sinter any glass-containing material inthe layers thus, densifying the thick film layer.

Fired thickness of the film can vary depending on screen mesh size,content of glass and solid % in paste, but typically ranges from 10 μmto 25 μm. To prevent readsorption of volatiles (and de-activation of thezeolite), the firing (or densifying) step is often conducted in acontrolled atmosphere void of moisture and other gases, such as undervacuum. The firing step is usually performed immediately prior tosealing the device into the hermetic container unless the fired getteris stored in an atmosphere void of moisture and/or other gases.Depending on processing procedure, an additional firing at 400-550° C.for activation of getter may be required to evaporate absorbed moisture.

The humidity sensitivity of fired getter thick films was evaluated byexposing the getter composition to changing humidity conditions, such asdifferent humidity levels, exposure time, and temperature. The thermalgravimetry analysis (TGA) analysis was used to quantify the weight lossup to a certain temperature. The evaluation results showed the identicaldecreasing tendency of weight loss with increasing the glass content. Inaddition, it revealed that most of humidity absorption occurred within arelatively short time of less than 1 hour of exposure.

The present invention will be discussed in further detail by givingpractical examples. The scope of the present invention, however, is notlimited in any way by these practical examples.

EXAMPLES Examples 1-3

A series of bismuth silicate-based glass compositions that have beenfound to be suitable in the present invention for application to thickfilm getter paste are shown in Table 1. The examples particularlyillustrate silicate glasses containing more than 69 wt. % bismuth.

All glasses were prepared by mixing raw materials and then melting in aplatinum crucible at 1100-1400° C. The resulting melt was stirred andquenched by pouring on the surface of counter rotating stainless steelrollers or into a water tank. The glass powders prepared for theinvention were adjusted to a 2-5 μm mean size by wet or dry millingusing alumina ball media prior to formulation as a paste. The wet slurryafter milling was dried in a hot air oven and deagglomerated by thesieving process.

TABLE 1 Glass composition in weight % Ex. # 1 2 3 SiO₂ 7.1 15.8 3.5Al₂O₃ 2.1 2.6 3.5 Bi₂O₃ 69.8 81.6 82.0 B₂O₃ 8.4 CaO 0.5 ZnO 12.0 PbO11.0

Examples 4-7

Table 2 represents a series of lead or phosphorous-based silicate glasscompositions that have been found to suitable for this getterapplication. The same processing as described in Example 1-3 was appliedto make glass frit powders of a 2-5 μm mean size.

TABLE 2 Glass composition in weight % Ex. # 4 5 6 7 SiO₂ 14.8 9.1 35.61.3 Al₂O₃ 0.8 1.4 7.6 19.8 PbO 66.0 77.0 47.0 B₂O₃ 11.8 12.5 6.8 ZnO 6.6SnO₂ 9.8 Na₂O 9.8 Li₂O 3.7 P₂O₅ 42.23 NaF 16.41

Examples 8-18

Thick film getter paste was prepared by mixing glass and desiccant (4 Aor 13x powder) material (molecular sieve) with organic media based onthe mixture of Texanol® solvent and ethyl cellulose resin. Table 3represents the examples of thick film compositions containing 5-76%glass. Different levels of glass relative to desiccant were applied foroptimized performance. Particularly, examples 10 and 18 demonstratethick film compositions containing only glass and organic media.Changing the content of solvent was necessary to adjust paste viscosityand film thickness for different deposition methods.

The getter paste was printed using a 200 mesh screen on a glass lidsubstrate based on soda-lime silicates, dried at 120° C. for solventevaporation, and then fired at a peak temperature of 450-550° C. for 1-2hours in a box furnace. Some samples were also processed at 550° C. for1 hour using a conveyor furnace with a heating/cooling profile of about6 hours. The printing/firing step was repeated to generate thickergetter films when needed. The fired thickness of single-printed thickfilm ranged from 10 μm to 25 μm, depending on paste viscosity and screenmesh size.

The thick film fired dense and showed good adhesion with the glasssubstrate. No cracking or blistering was observed on the surface offired thick films. Good thickness uniformity of fired film was keptwithin +/−2 μm regardless of paste composition.

The densification degree of fired samples depended on the relativecontent of glass. A higher content of glass was desirable for betterdensification of thick films and stronger bonding to the glass lidsubstrate.

TABLE 3 Thick film getter composition in weight % Ex. # 8 9 10 11 12 1314 15 16 17 18 Glass. # 1 1 1 2 3 4 5 5 6 6 7 Glass 5.4 14.0 75.8 19.619.6 17 8.9 19.6 8.9 19.6 75.8 Molecular 54.1 46.5 — 39.2 39.2 42.6 44.439.2 44.4 39.2 — Surfactant 1.1 0.9 — 0.8 0.8 0.9 0.9 0.8 0.9 0.8 —Binder Resin 1.0 0.9 1.4 0.7 0.7 0.8 0.8 0.7 0.8 0.7 1.4 Solvent 38.437.7 22.8 39.7 39.7 38.8 45 39.7 45 39.7 22.8

Examples 19-25

Fired thick film samples were evaluated to quantify the degree ofmoisture absorption after exposure to certain humidity environment.Table 4 shows the absorption performance of selected getter samplesbased on the thermal gravimetry analysis (TGA) results. The absorptionresults were expressed as weight loss of thick films up to a giventemperature. The TGA samples were prepared by firing at 550° C. for 2hours and exposing to an 85° C./85% RH humidity condition for 1 hour. Noadditional firing for activation was used for the TGA analysis.

The moisture absorption of getter paste was found to depend strongly ontemperature and relative content of glass as seen in the examples 19-24.The weight loss proportionally increased with increasing temperature asexpected, but it tends to show the saturation of weight loss after 400°C. Increasing the content of glass decreased drastically the percentageof weight loss. For example, a weight loss of 16% was detected for thesample containing 28.6% glass according to TGA up to 400° C., while only0.4% weight loss for the 66.7% glass composition. This tendency isreasonable when considered the fact that the absorption capability isdominantly determined by the relative content of desiccant materials.The glass is believed to generally act as an inorganic binder and/or anadhesion promoter while it degrades the capability of water absorptionas the glass example 1 basically does not have water absorptioncapability as seen in Example 24.

On the other hand, it is interesting to demonstrate that Example 25based on phosphorous glass (Example 7) without molecular sieve showedconsiderable amounts of water absorption depending on firingtemperature. It indicates that pure glass-based materials can be usedfor moisture-gettering purpose especially for highly moisture-sensitivedevices. It is noticeable that Example 25 showed better water absorptionpercentages compared to Example 23 containing 33 wt. % molecular sieve.

TABLE 4 Thick Film getter composition in weight % (based on solid %) andweight loss in % Ex. # 19 20 21 22 23 24 25 Glass # 1 1 1 1 1 1 7 Glass28.6% 37.5% 44.5% 50.0% 66.7%  100%  100% Molecular Sieve 71.4% 62.5%55.5% 50.0% 33.3% Weight Loss by TGA Up to 100° C. 4.0% 5.5% 3.7% 1.3%0.2%   0%  0.4% Up to 200° C. 13.0% 13.5% 10.7% 3.6% 0.4%   0%  1.2% Upto 300° C. 16.0% 15.0% 12.5% 4.1% 0.5% 0.01%  1.5% Up to 400° C. 16.4%15.5% 12.8% 4.2% 0.6% 0.05%  1.6% Up to 500° C. 16.6% 15.5% 13.0% 4.3%0.6% 0.06%  1.7%

1. An article comprising a substrate bonded with a sintered thick filmhygroscopic getter composition, said thick film hygroscopic gettercomposition consisting of: (a) hygroscopic bismuth silicate-based glassfrit comprising more than 69 wt. % Bi₂O₃ and 0.8-30 wt. % Al₂O₃dispersed in (b) organic medium comprising an organic polymeric binderand a volatile organic solvent, wherein the organic polymeric binder isselected from the group consisting of ethyl cellulose, ethylhydroxyethylcellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins,polymethacrylates of lower alcohols, monobutyl ether of ethylene glycolmonoacetate, and mixtures thereof; wherein the volatile organic solventis selected from the group consisting of ethyl acetate, terpenes,kerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate,hexylene glycol, high boiling alcohols, alcohol esters, and mixturesthereof, and; wherein said thick film hygroscopic getter composition isbonded to the substrate by depositing the thick film hygroscopic gettercomposition onto the substrate, followed by volatilizing the organicmedium and sintering the glass frit to form an essentially non-porouslayer of thickness from 10 μm to 25 μm.
 2. The article of claim 1comprising a hygroscopic getter composition, wherein the organic mediumcomprises 30-55% weight percent, based on the total weight of thecomposition.
 3. The article of claim 1 comprising a hygroscopic gettercomposition, wherein the hygroscopic glass frit comprises 5-55 weightpercent, based on the total weight of the composition.
 4. The article ofclaim 1 comprising a hygroscopic getter composition, wherein thehygroscopic glass frit comprises, based on weight of the hygroscopicglass frit 1-50% SiO₂, 0-80% B₂O₃, 0-90% PbO, 0-90% P₂O₅, 0-60% Li₂O,0-10% K₂O, 0-10% Na₂O, and 0-30% MO where M is selected from Ba, Sr, Ca,Zn, Cu, Mg and mixtures thereof.
 5. The article of claim 1 comprising ahygroscopic getter composition, wherein the hygroscopic gettercomposition has been fired at 400° C. to 650° C. and is attached to asubstrate.